Emulsion polymerization is currently used in the commercial production of various polymers and copolymers including polymers and copolymers of vinyl acetate, various acrylates and methacrylates, acrylonitrile and methacrylonitrile, styrene and other vinyl substituted aromatics, vinyl chloride, vinylidene chloride, and the like. Emulsion polymerization or copolymerization generally occurs in an aqueous medium via free radically initiated addition polymerization to yield an aqueous dispersion of polymer particles, referred to as a latex or suspension, which can be either used directly or compounded with other ingredients for use as a coating, adhesive, paint sealant, etc., or the polymer particles can be separated from the dispersion such as by coagulation, spray drying, precipitation, etc., to yield a dry, powdered, thermoplastic resin. Relatively high monomer conversions such as 90 percent or higher are generally achieved during commercial production of polymer dispersions. The amount of unreacted or residual monomer remaining in the latex at the conclusion of the polymerization process is often as high as about 2 weight percent. Steam stripping, nitrogen and air have been conventionally used to physically remove most of the residual unsaturated monomers from the dispersion in order to meet existing health and environmental regulations relating to the control of volatile organic compounds. For example, conventional steam stripping is practical for recovering most of the unreacted vinyl chloride monomer from a polyvinyl chloride latex for reuse, and for reducing the residual monomer content from about 2 weight percent by weight to about 0.1 weight percent based upon the total weight of the aqueous dispersion. Nitrogen stripping provides a practical method for recovering additional residual monomer and for further reducing the vinyl chloride content in the latex to about 2 weight percent to 0.07 weight percent. Further reductions in the level of residual monomer are not economically or commercially practical using conventional stripping processes. Accordingly, to meet impending stricter health and environmental regulations, it will be necessary to develop new techniques which can be used to achieve significant further reductions in the amount of residual monomer contained in latices without significantly adding to the production costs or adversely affecting the properties and utility of the latices.
Whereas there are incentives to reduce the levels of unsaturated monomer, one must achieve such reductions in the absence of detrimental results to the desired product, namely, the saturated polymer. For example, any reduction must not destabilize the dispersion of polymer or significantly offset the desired molecular weight of the polymer subsequent to the polymerization reaction.
Various methods of reducing the amount of unsaturated monomer in the dispersion below the levels which are economically feasible utilizing conventional stripping techniques have been proposed. Such methods include chemical absorption, condensation and adsorption, biofiltration and various incineration techniques. These proposed methods are generally commercially unacceptable on account of a combination of being uneconomical, inefficient, difficult to control, and unreliable.